Environmental Matters - Environmental Remediation Costs (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Environmental Exit Cost [Line Items]
Remediation costs incurred	$ 5	$ 10	$ 19	$ 35
Insurance recoveries received			5
CECONY [Member]
Environmental Exit Cost [Line Items]
Remediation costs incurred	2	10	12	30
Insurance recoveries received			$ 5